Natural deep eutectic solvents: cytotoxic profile | SpringerLink

Hayyan et al. SpringerPlus (2016) 5:913
DOI 10.1186/s40064-016-2575-9
Open Access
RESEARCH
Natural deep eutectic solvents: cytotoxic
profile
Maan Hayyan1,2* , Yves Paul Mbous1,3, Chung Yeng Looi5, Won Fen Wong4, Adeeb Hayyan1,6,
Zulhaziman Salleh1,3 and Ozair Mohd‑Ali7
Abstract The purpose of this study was to investigate the cytotoxic profiles of different ternary natural deep eutectic solvents
(NADESs) containing water. For this purpose, five different NADESs were prepared using choline chloride as a salt,
alongside five hydrogen bond donors (HBD) namely glucose, fructose, sucrose, glycerol, and malonic acid. Water was
added as a tertiary component during the eutectics preparation, except for the malonic acid-based mixture. Coinci‑
dentally, the latter was found to be more toxic than any of the water-based NADESs. A trend was observed between
the cellular requirements of cancer cells, the viscosity of the NADESs, and their cytotoxicity. This study also highlights
the first time application of the conductor-like screening model for real solvent (COSMO-RS) software for the analysis
of the cytotoxic mechanism of NADESs. COSMO-RS simulation of the interactions between NADESs and cellular mem‑
branes’ phospholipids suggested that NADESs strongly interacted with cell surfaces and that their accumulation and
aggregation possibly defined their cytotoxicity. This reinforced the idea that careful selection of NADESs components
is necessary, as it becomes evident that organic acids as HBD highly contribute to the increasing toxicity of these neo‑
teric mixtures. Nevertheless, NADESs in general seem to possess relatively less acute toxicity profiles than their DESs
parents. This opens the door for future large scale utilization of these mixtures.
Keywords: Natural deep eutectic solvents, Green solvents, Ionic liquids, Choline chloride, Cytotoxicity, Cancer cell
line, COSMO-RS
Background
The use of volatile organic compounds (VOCs) has left in
its wake countless considerations, most of which associated with safety and toxicity issues (Bushnell et al. 2007).
Therefore, for various chemical and biological industries,
one of the most pressing concern remains the development of ‘greener’, lower cost, and more efficient solvents.
The discovery of ionic liquids (ILs) seemed to provide
the solution to this predicament. ILs being low-melting
organic salts composed of ionic species, which are often
found in liquid state at temperatures below 100 °C (Ru
and Konig 2012). ILs are characterized by a number of
attractive properties such as high thermal stability, nonflammability, high solvability, chemical stability, low
*Correspondence: [email protected]
1
University of Malaya Centre for Ionic Liquids (UMCiL), University
of Malaya, 50603 Kuala Lumpur, Malaysia
Full list of author information is available at the end of the article
volatility and tunability (Sowmiah et al. 2009). Notwithstanding their impressive contributions in various processes such as biotransformations (Domínguez de María
and Maugeri 2011), biodiesel production (Ullah et al.
2015), extraction processes (Pereira et al. 2015), active
pharmaceutical ingredients (Ferraz et al. 2011), and biomass treatment (da Costa Lopes et al. 2013); the use of
ILs has often been marred with issues pertaining to high
cost of synthesis, purification requirements, and toxicity
(Gorke et al. 2010). The ambiguity surrounding their use
has led to the emergence of an alternative class of solvents called deep eutectic solvents (DESs). As eutectics,
DESs exhibit freezing points lower than those of their
chief components (salt and HBD) (Smith et al. 2014).
This depression in temperature is the result of the charge
delocalization occurring via hydrogen bonding between
the anion of the salt and the HBD (Ru and Konig 2012).
DESs have been introduced as viable replacements of ILs
because, on top of possessing similar physicochemical
© 2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License
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Hayyan et al. SpringerPlus (2016) 5:913
aspects, they offer several other advantages, such as the
low cost of their starting materials, ease of preparation,
and no waste generation (Tang and Row 2013). As a
result, they have been used in a wide number of applications such as extraction processes (Qi et al. 2015), biotransformations (Wu et al. 2014), nanoparticles assembly
(Wagle et al. 2014), preservation of biomolecules (Dai
et al. 2015), upstream and downstream biodiesel processing (Hayyan et al. 2010, 2013a, 2014), electrodeposition (Ru et al. 2015), and organic synthesis (Zhang et al.
2012).
The most encountered DESs are based on ChCl, which
revolves around choline. Choline is a known component
of Vitamin B, and plays important metabolic functions
(Florindo et al. 2014). The qualifications of choline as a
safe ingredient led to the assumption that DESs possess
negligible toxicity/cytotoxicity profiles.
However, the leading cytotoxic assessments of DESs
showed that they can be lethal to both terrestrial and
marine organisms (bacteria and brine shrimp) (Hayyan
et al. 2013b, c). Ammonium-based DESs, namely:
[ChCl]-[Glycerol], [ChCl]-[Ethylene glycol], [ChCl][Triethylene glycol], [ChCl]-[Urea]; and methyltriphenyl phosphonium bromide (MTPB)-based DESs such
[MTPB]-[Glycerol], [MTPB]-[Ethylene glycol], and
[MTPB]-[Triethylene glycol], were used during these
investigations. Both ammonium and phosphoniumbased DESs were found toxic to brine shrimp, but only
phosphonium-based DESs exhibited bacterial toxicity.
The cytotoxicity of both ammonium and phosphoniumbased DESs was higher than those of their individual
components (Hayyan et al. 2013b, c). Therefore, the
authors concluded that although ChCl and MTPB salts
are not devoid of toxicity; their association with a HBD
during DESs’ preparation increases the eutectics’ cytotoxicity considerably. This conclusion has been recurrent throughout several cytotoxic assessments of DESs
(Hayyan et al. 2015; Radošević et al. 2015; Wen et al.
2015).
The HBD has proven to be of significant importance with regards to DESs’ cytotoxic profiles. Recently,
Radošević et al. (2015) examined the cytotoxic profile of
[ChCl]-[Glucose], [ChCl]-[Glycerol], and [ChCl]-[Oxalic
acid] on channel catfish ovary (CCO) fish cell line and
the human breast adenocarcinoma cell line (MCF-7).
Their results showed that the [ChCl]-[Oxalic acid] DES
exhibited a significantly higher toxicity (EC50 < 5 mM)
compared to the remaining ChCl-based DESs (EC50
> 10 mM). These results reinforced the importance of
a careful selection of the HBD prior DESs synthesis. In
yet another study, among four ChCl-based DESs namely,
[ChCl]-[Urea], [ChCl]-[Acetamide], [ChCl]-[Ethylene
glycol], and [ChCl]-[Glycerol]; the [ChCl]-[Ethylene
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glycol] DES was shown to have the highest toxicity (Wen
et al. 2015).
The cytotoxic mechanism of DESs outlines an
increased membrane porosity due to the continuous
DESs’ induced damage of the plasma membrane (Hayyan
et al. 2015). Accordingly, upon penetration, DESs’ species effectively contribute to the increase of reactive oxygen species (ROS) concentrations, hereby subjecting the
cell to increasing oxidative stress. The end-point of this
cascade involves the complete destruction of the cell
through apoptosis (Hayyan et al. 2015).
The key to producing less toxic DESs may reside in the
use of materials of natural origin. Recently, Choi et al.
(2011) revealed that a number of primary metabolic
substances (e.g. sugars, amino acids, choline, and some
organic acids) form intracellular eutectic mixtures to
assist plants during specific developmental stages (germination, cryopreservation). These eutectics provide
cells with a third type of solvent/media, completely different from lipids and water. The presence of these eutectic mixtures—termed natural deep eutectic mixtures
(NADESs)—intracellularly, entails their cellular tolerance and presume safer cytotoxic profiles. Recent studies
have provided a list of the composition of these mixtures
as well as their molar ratios (Choi et al. 2011; Dai et al.
2013).
One of the fundamental precepts of this class dictates that if cellular media produce NADESs, the propensity for cytotoxicity must be minimal. Paiva et al.
(2014) briefly investigated the cytotoxic profile of several NADESs namely, [ChCl]-[Glucose], [ChCl]-[Citric
acid], [ChCl]-[sucrose], [ChCl]-[Tartaric acid], [ChCl][Xylose], [Citric acid]-[Glucose], [Citric acid]-[Sucrose],
[Glucose]-[Tartaric acid] at various molar ratios. Using
fibroblast-like cells, the authors assessed cellular viability
following NADESs’ treatment. The results pointed once
again to the role of the HBD (organic acids) as major
enhancer of cytotoxicity, because the most toxic NADESs
were [Glucose]-[Tartaric acid], [ChCl]-[Tartaric acid],
[ChCl]-[Citric acid], and [Citric acid]-[Glucose]. Understanding the various interactions or forces resulting from
the association of NADESs chief elements can provide
further elucidation of their resulting cytotoxic profiles.
A recent study has shown that NADESs physical properties can be tailored by adding water as a tertiary component. The authors showed that the strong hydrogen
interactions within NADESs (accounting for their high
viscosities) could be reduced upon addition of water
(<50 % v/v). In fact, the resulting viscosities were found
to be as low as those of water and other common VOCs
(Dai et al. 2015). Consequently, water-based NADESs
may represent yet another alternative to DESs of high viscosities, poor conductivities and higher toxicities.
Hayyan et al. SpringerPlus (2016) 5:913
Hence, one of the objectives of this study was to investigate the effect of water on the cytotoxic profiles of
NADESs by exploring the impact ternary NADES systems (containing water) have on several cancer cell lines.
The understudied eutectics were ChCl-based NADESs
prepared using five HBDs, namely, fructose, glucose,
sucrose, glycerol, and malonic acid. Alternatively, we
aimed to compare the effect of biomaterials against that
of organic acids on mammalian cells. Lastly, we sought
to model the interactions between NADESs and cellular membranes using COSMO-RS computational
methodology in order to understand NADESs cytotoxic
mechanism.
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Methods
Chemicals and materials
The chemicals used for NADESs preparation were purchased from both Merck (sucrose, fructose and glucose)
and Sigma-Aldrich (ChCl and malonic acid), Fig. 1. The
purity of these chemicals was higher than 99 %.
The human cervical cancer cell line (HelaS3), the
human ovarian cancer cell line (CaOV3), and the mouse
skin cancer cell line (B16F10) were purchased from the
American Type Culture Collection (ATCC, Manassas,
VA). The human breast cancer cell line (MCF-7) was
obtained from Cell Lines Service (300273; Eppelheim,
Germany).
Fig. 1 Structure of the individual components of the NADESs used in this study
Hayyan et al. SpringerPlus (2016) 5:913
Page 4 of 12
Both the Dulbecco’s Modified Eagle Medium (DMEM)
and the Roswell Park Memorial Institute medium (RPMI
1640) were obtained from Life Technologies, Inc., Rockville, MD. Fetal bovine serum (FBS) was supplied by
Sigma-Aldrich.
Preparation of NADESs
Table 1 illustrates the composition, molar ratios and symbols of the NADES used throughout this study. The preparation method is similar to those previously described in
the literature (Hayyan et al. 2013c).
Cell culture
HelaS3, CaOV3, and B16F10 were grown in DMEM
supplemented with 10 % FBS, 1 % penicillin and streptomycin. MCF-7 were grown in RPMI supplemented
with 10 % FBS, 1 % penicillin and streptomycin. The cells
were kept in culture flasks inside an incubator providing a humidified atmosphere of 37 °C, with 5 % CO2. The
cells were grown to a necessary confluence of 70–80 %,
necessary for the 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT) viability assay.
MTT viability assay
The MTT cell viability assay was performed as previously described (Hayyan et al. 2015). The IC50 values were
obtained from an average of at least 3 independent experiments. The standard error of the mean (SEM) derived
from the repeated experiments were used to derive the
variations from the average IC50 values. The statistical
analysis was performed using Graph Pad Prism 5 software.
Statistical significance was defined when P < 0.05.
Computational methodology for COSMO‑RS
Molecular geometry optimization
The geometry optimization of all species involved in
this study was performed using the Turbomole program package. Using this program, the basic structure
of the target molecule was drawn first. After which,
Table 1 Compositions, symbols, and molar ratios of the
NADESs used in this study
Salt
HBD
Add-on Molar ratio Appearance
Symbol
ChCl Fructose
Water
5:2:5
Moderately
viscous liquid
NADES1
ChCl Glucose
Water
5:2:5
Moderately
viscous liquid
NADES2
ChCl Sucrose
Water
4:1:4
Moderately
viscous liquid
NADES3
ChCl Glycerol
Water
1:2:1
Lightly viscous
liquid
NADES4
1:1
Moderately
viscous liquid
NADES5
ChCl Malonic acid –
geometry optimization was performed at the Hartree–
Fock level and 6-31G* basis set. The generation of
cosmo file was then conducted through a single-point
calculation by using DFT with Becke–Perdew and
the Triple-ζ Zeta Valence Potential (TZVP) basis set.
Finally, the cosmo files were exported to the COSMOthermX program with parameterization BP_TZVP_
C30_1301.ctd.
DES representation in COSMOtherm‑X
Since a single DES is composed of more than one molecule, employing its representation method in the
COSMOtherm-X program is crucial. In this study, the
electroneutral approach was adopted, whereby the DES
was represented in COSMO-RS according to the mole
composition of their constituents shown in Table 1 [the
salt cation, salt anion, and hydrogen bond donor (HBD)].
Membrane phospholipids were designed according to the
same principle; that is using the most basic composition
of their constituents (Table 2).
Results and discussion
The cytotoxicity of the five understudied NADESs was
assessed on various human and mice cancer cell lines,
namely, HelaS3, CaOV3, MCF-7, and B16F10. Table 3
illustrates the IC50 values obtained. The results indicate
the following decreasing order of toxicity for HelaS3,
MCF-7, and B16F10 cell lines: NADES5 > NADES3 > NA
DES1 > NADES2 > NADES4. In CaOV3 case, NADES2
was more toxic than NADES1 resulting in a slightly different trend: NADES5 > NADES3 > NADES2 > NADES1 > NADES4. However, if the SEM of the IC50 values are
included, the resulting IC50 intervals of both NADES1
(198.5–213.5 mM) and NADES2 (185.5–200.5 mM) overlap, as the end values are close to one another.
Overall, we noticed a trend between NADESs’ cytotoxicity and various factors namely the cellular requirements
of cancer cells, the physical properties of NADESs (especially viscosity); the addition of water; and the nature
of NADESs’ raw materials as well as their interactions
with the different functional groups present on the cell
surface.
The merits of most DESs stem from the qualifications of ChCl; with specific referral to the metabolism
and function of choline in mammalian cells. Choline is
the preferred cellular raw material used for the synthesis of cellular membranes phospholipids, namely phosphatidylcholine, and sphingomyelin (Lodish et al. 2000;
Plagemaen 1971). Consequently, ChCl has been classified as a salt of relatively safe profile (although high
intake is associated with adverse conditions). However,
the DESs cytotoxic profiles obtained thus far do not
share the negligible cytotoxic labeling of ChCl. This has
Hayyan et al. SpringerPlus (2016) 5:913
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Table 2 Composition of cellular membrane and ratio (Harwood and Weselake 2015)
Cell membrane elements
Composition and ratio
Fatty acid
Alcohol
Metabolites
Functional group
Phosphatidylcholine
Palmitic acid (2)
Glycerol (1)
Choline (1)
Phosphate (1)
Phosphate (1)
Phosphatidylethanolamine
Linoleic acid (1); palmitic acid (1)
Glycerol (1)
Ethanolamine (1)
Phosphate (1)
Phosphate (1)
Phosphatidylserine
Stearic acid (1); cervonic acid (1)
Glycerol (1)
Serine (1)
Phosphate (1)
Phosphate (1)
Sphingomyelin
Oleic acid (1)
–
Choline (1)
Phosphate (1)
Phosphate (1)
Glycolipids
Oleic acid (1)
–
Glucose (1)
Sphingosine (1)
Phosphate (1)
Table 3 IC50 of the studied NADESs on various cell lines
Solvent
IC50 (mM)
Hela S3
CaOV3
MCF-7
B16F10
NADES1
177 ± 7.3
206 ± 7.5
127 ± 9.22
195 ± 7.7
NADES2
182 ± 7.6
193 ± 7.5
186 ± 7.9
211 ± 8
NADES3
166 ± 5.8
154 ± 5.6
150 ± 5.5
136 ± 5.7
NADES4
427 ± 11
483 ± 11
457 ± 11
340 ± 10.3
NADES5
20 ± 8.4
15 ± 8.2
35 ± 8.3
35 ± 8.8
prompted the examination of the role of the HBD in
these profiles.
From a cellular perspective, fructose, glucose, sucrose
(50 % glucose and 50 % fructose), and glycerol are essential carbohydrates whose metabolism provide energy
required for various cellular functions. Upon adsorption, fructose and glucose undergo glycolysis if energy is
needed, or are stored as glycogen. The glycolytic pathway
for these molecules leads to either the pentose phosphate
pathway (for nucleic acid synthesis), the mitochondrial
tricarboxylic acid pathway (for energy production), or
de novo lipogenesis (for fatty acids synthesis). Cancer
cells, especially, require more energy than normal cells,
given their abnormal and exponential growth features.
Therefore, they use a higher amount of energy or energy
sources (glucose and fructose) for metastasis, growth,
invasion and migration purposes (Port et al. 2012; Santos
and Schulze 2012).
Likewise, glycerol is the precursor of triglycerides and
phospholipids. It is activated by a phosphorylation reaction and forms glycerol-3-phosphate (G-3-P), which is
then involved in the carbohydrate and lipid metabolism.
Alternatively, glycerol also functions as a shuttle of electrons from the cytosol to the mitochondria by regenerating NAD+ from NADH (Laforenza et al. 2016). In both
normal and cancer cell lines, glycerol can be used for
gluconeogenesis, although the main metabolite used for
that purpose is different; probably glycogen. Nevertheless, there is evidence that in cancer cells, a higher than
normal plasma concentration of glycerol (comparable in
this case to NADES4 treatment) contributes to increased
glycerol turnover for gluconeogenesis and lipogenesis
(Liu et al. 1995; Lundholm et al. 1982). Judging from
these facts, a higher cellular tolerance of these carbohydrates-based eutectics is expected, and this is reflected by
the IC50 values recorded for NADES1, NADES2, NADES3
and NADES4.
In contrast, NADES5 which boasts of organic acid as
raw material, is the most lethal mixture. Dai et al. (2013)
listed NADES5 as eutectic used by plants for developmental or metabolic purposes. Although this is valid
for certain plants tissues—where malonic acid accounts
for as much as 4 % of the dry weight and up to 50 % of
the total acid content and may be actively used during
nitrogen assisted symbiosis or abiotic stress as a defense
chemical; the scenario might be slightly different for
mammalian cells (Kim 2002). Indeed, in mammalian systems, malonic acid is known to stall the Krebs cycle by
inhibiting succinic dehydrogenase (mitochondria complex II); a crucial enzyme for the citric acid cycle and
the electron transport chain (Hosoya and Kawada 1958).
Consequently, this paralyzes ATP synthesis. Moreover,
malonate is known to disrupt glycogenesis, lipid synthesis and carbon dioxide production during glycolysis
(Hosoya et al. 1960). It comes as no surprise that calls
have been made for malonate to be used as an anticancer agent. As a matter of fact, Fernandez-Gomez et al.
(2005) showed that malonate causes SH-SY5Y neuroblastoma mitochondrial failure by inducing a rapid build-up
of ROS, which overwhelms mitochondrial antioxidant
capacity, ultimately leading to cellular apoptosis.
This shows that with regards to the HBD, the inclusion of organic acids seem to increase the overall toxicity
Hayyan et al. SpringerPlus (2016) 5:913
of NADESs. This is consistent with the other cytotoxic
reports on DESs/NADESs (Paiva et al. 2014; Radošević
et al. 2015; Zhao et al. 2015).
Zhao et al. (2015) observed that NADESs with organic
acids as HBDs had a low pH (<6.5); when the optimal
growth range for mammalian cells is 7.0–7.4. This change
in environmental conditions is partially responsible for
the high toxicity of NADES5.
DESs investigations led to a similar observation. For
instance, Radošević et al. (2015) observed the formation of intracellular calcium oxalate crystals following
[ChCl]-[Oxalic acid] DES treatment of CCO and MCF-7
cell lines. Another perspective on organic acids as HBDs
was shown by Paiva et al. (2014). The authors examined
NADESs toxicity towards fibroblasts like-cells (L929) and
reported that the most lethal solvents also had organic
acids as HBDs (i.e. tartaric acid and citric acid). However,
it must be noted that the solvents with the highest viability, also had organic acids as ingredients, although the
remaining constituent was another HBD (sucrose) and
not a salt (ChCl) (Paiva et al. 2014). It might be that the
devastating effect of organic acids in NADESs is better
countered with the use of biomaterials (e.g. sugars).
The arguments above do not presume to provide a
complete understanding of the reasons behind the variation in IC50 values; but serve to highlight that safer
NADESs can be obtained by using biomaterials of cellular necessity. Of course, the interactions of these mixtures
and their aggregation on cellular membranes as well as
the neoteric properties of NADESs, remain aspects to be
investigated. Meanwhile, physical properties of NADESs
can also be used to better appreciate the obtained cytotoxic values.
Table 4 lists the known viscosities values at 30 °C of
the understudied NADESs. The values for NADES1,
NADES2, and NADES5 were obtained from a recent
study by Zhao et al. (2015). The viscosities of NADES3
and NADES4 were measured independently during this
study. Sorting out viscosities in a decreasing order (NAD
ES3 > NADES5 > NADES2 > NADES1 > NADES4) reveals
that they form a series almost similar to the cytotoxicity
trend above.
According to Table 4, NADES5 and NADES3 possess
the highest viscosities at 30 °C (respectively 616 and
853.3 mPa s). It is no surprise that they also possess the
lowest IC50 values on average across all examined cells, as
high viscosity is often associated with increased lethality.
Despite being less viscous than NADES3, NADES5 was
identified as the most toxic material tested, with an IC50
interval (15 ≤ IC50 ≥ 35 mM) at least three times lower
than NADES3’ interval (136 ≤ IC50 ≥ 166 mM).
In a separate study, upon testing numerous DESs and
NADESs (including similar NADES1, NADES2, and
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Table 4 Viscosities of the understudied NADESs
NADES
Molar ratio
Viscosity (mPa s)
References
NADES1
5:2:5
584
Zhao et al. (2015)
NADES2
5:2:5
598
Zhao et al. (2015)
NADES3
4:1:4
853.3
–
NADES4
1:2:1
36
–
NADES5
1:1
616
Zhao et al. (2015)
NADES5 used in this study) on several bacteria species
(i.e. Escherichia coli, Staphylococcus aureus, Salmonella
enteritidis, Listeria monocytogenes), Zhao et al. (2015)
also identified NADES5 as one of the most toxic mixture.
In contrast, NADES1 and NADES2 were found to be noninhibitory to all studied bacteria. Moreover, the most
toxic DESs and NADESs ([ChCl]-[Toluenesulfonic acid],
[ChCl]-[Oxalic acid], [ChCl]-[Levulinic acid], [ChCl][Malonic acid], [ChCl]-[Malic acid], [ChCl]-[Citric acid],
[ChCl]-[Tartaric acid] all included organic acids as HBDs
(Zhao et al. 2015). Just as in our study, where, the malonic
acid based NADES was the most toxic. This stipulates
that despite resulting in high viscosities (compared to
water and VOCs), NADESs composed of sugars are relatively less dangerous to biological machinery than those
composed of organic acids.
On the other hand, NADES4 exhibited the lowest viscosity at 30 °C (36 mPa s) as well as the highest cytotoxicity values. With reference to Table 4, NADES2 is slightly
more viscous than NADES1. Their cytotoxicity values
are roughly similar if the SEM is taken into account. Viscosity or micro-viscosity (in cellular terms) is an important property to consider in intracellular activities. Not
only does it affect diffusion within biological systems,
but it is also involved during processes such as protein–
protein interactions, transportation of small solutes
and macromolecules, and signal transduction in living
cells. The local micro-viscosity in cells ranges from 1 to
400 mPa s (Liu et al. 2014). The highest viscosity values
(≥200 mPa s) are usually associated with the microviscosity in the hydrophobic domains of living cells (lipid
bilayers of cell surfaces); whereas, values between 1 and
3 mPa s are attributed to the aqueous phases of the cellular cytoplasm (Juneidi et al. 2015; Kuimova et al. 2009).
A variation or a disturbance of these homeostatic values leads to the onset of various diseases (atherosclerosis, diabetes) as well as cell death (Deliconstantinos et al.
1995; Nadiv et al. 1994). The understudied NADESs possess viscosities higher than 500 mPa s; with the exception
of NADES4 (36 mPa s), which happens to be the least
toxic mixture tested (with IC50 values 1.6–32 times lower
than the other NADESs). Hence, it is not difficult to perceive the substantial influence that these highly viscous
Hayyan et al. SpringerPlus (2016) 5:913
materials can have on cells. Just like DESs perforate cellular membranes (Hayyan et al. 2015), NADESs can probably enhance cellular membrane permeability. As such,
the introduction of such viscous substances in cellular
medium can result in a major variation of cytoplasmic
microviscosity, and eventually lead to cell death.
Based on the available knowledge of DESs, the high
viscosities of NADESs originate from the rigidity of their
supramolecular complexes reposing on a strong hydrogen bond network. It entails that the disruption of this
network will affect the viscosity of NADESs. In fact, Dai
et al. (2015) recently provided evidence of the progressive
rupture of this hydrogen bond network upon addition
of water. The authors also showed that the supramolecular complexes of NADESs remain intact if the volume
of added water is less than 50 %. Pass this threshold, the
resulting mixture consists merely of dissociated NADESs
ingredients. This is a consequence of the complete rupture of hydrogen bonds stabilizing NADESs. The fact that
the entire NADES structure repose on hydrogen bonds
means that their progressive breakdown simultaneously
induces a change in their physicochemical properties.
Accordingly, Dai et al. (2015) reported a decrease in viscosity from 397 to 7.2 mPa s, following the addition of
25 % of water to [ChCl]-[Glucose]-[Water] (in this case
NADES2).
This argument is further justified by the fact that we
have independently recorded a value of 36 mPa s for
NADES4 ([ChCl]-[Glycerol]-[Water]) viscosity; whereas
Zhao et al. (2015) recorded a value of 177 mPa s for the
[ChCl]-[Glycerol] DES (both at 30 °C). The effects of
water can also be acknowledged through the observation
that of all the understudied NADESs, the most toxic and
most viscous was the one prepared without the use of
water (NADES5). Moreover, Dai et al. (2015) stated that
the water activity of NADESs increases with increasing
water content (or after water addition). Consequently,
the polarity of the eutectics after addition of water may
mimic that of water itself. This may influence the interactions of these solvents with cell surfaces.
The importance of these interactions must be dully
underscored, as DESs have shown that they promote
cellular failure through an increase in membrane porosity (Hayyan et al. 2015). In order to have an idea of what
unfolds upon NADESs treatment on cell surface, we used
a computational methodology using the COSMO-RS
software. COSMO-RS is a very useful and fast tool for
the prediction of thermophysical and chemical properties of fluid mixtures (Klamt 2005). It is a model that
combines an electrostatic theory of locally interacting
molecular surface descriptors with statistical thermodynamics. Although mostly used to predict the thermodynamic properties of a mixture without prior experimental
Page 7 of 12
data; it can also be applied to life sciences and molecular
studies. Examples include the prediction of drug’s partition coefficients and the computation of proteins pKa
(Andersson et al. 2013; Buggert et al. 2009). Two major
steps are involved in the COSMO-RS prediction process.
The first step involves the creation of virtual conductor
surroundings for the molecule by using the continuum
solvation model. After performing the quantum chemical calculation through the density functional theory, a
screening charge density known as sigma (σ) forms on
the nearby conductor. The distribution of the screening
charge density on the surface of the molecule is then converted into a function of surface composition, known as
the σ-profile. The second step applies statistical thermodynamic principles to compute the molecular energy due
to the electrostatic misfit, hydrogen bond, and Van der
Waals interactions (Klamt and Eckert 2000). COSMORS can also be used to study the possible thermodynamic
behaviour of an individual component in a mixture and
its affinity or interactions with the other components
through the σ-profile and σ-potential, respectively. The
σ-profile describes the molecule polarity properties. Each
peak in the σ-profile plot for a molecule corresponds
to its constituent atoms depending on their screening
charge densities. The negative partial charges of atoms
cause positive screening charge densities, and vice versa.
Using the elements in Table 2, we modeled each of the
listed phospholipids and examined their interactions
with the NADESs. The sigma profiles of both phospholipids and NADESs are shown in Fig. 2. For comparison purposes, we only used phosphatidylcholine, as it
is the most common membrane phospholipid, and also
because, the phospholipids’ σ-profiles and potentials are
almost identical.
In the σ-profile, when the screening charge density is
lower than −0.0084 eÅ−2 or exceeds +0.0084 eÅ−2; the
molecule is considered sufficiently polar to induce hydrogen bonding.
Figure 2 is divided into three quadrants with corresponding σ values; the HBD region (σ < −0.0084 eÅ−2),
the nonpolar region (−0.0084 ≤ σ ≤ 0.0084 eÅ−2) and the
HBA region (σ > 0.0084 eÅ−2). Negative values represent
positive polarities and vice versa. Hence, the elements
in the HBD region of a molecule interact or attract elements in the HBA region (from another molecule) since
they are of opposite polarities. Looking at Fig. 2a, the
bulks of the peaks reside in the nonpolar region. This is
most likely due to the long hydrophobic fatty acid chains
making up these phospholipids and included during our
modeling. Compared to NADESs, phosphatidylcholine
σ-profile (similar to all other phospholipids) breadth is
narrower; this is usually indicative of a less polar character (Fig. 2b) (Mulyono et al. 2014).
Hayyan et al. SpringerPlus (2016) 5:913
a
140
Page 8 of 12
HBD Region
Non Polar region
HBA Region
120
p (σ)
100
80
Phosphatidylcholine
Phosphatidylethanolamine
Phosphatidylserine
Sphingomyelin
Glycolipids
60
40
20
0
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
-2
σ (eA )
NADES1
NADES2
NADES3
NADES4
NADES5
Phosphatidylcholine
b
140
120
p (σ)
100
80
60
40
20
0
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
-2
σ (eA )
Fig. 2 σ-profiles of phospholipids and NADESs. a Phospholipids σ-profiles are shown on their own, whereas b NADESs σ-profiles with phosphati‑
dylcholine for assessment purposes
The peaks located between +0.01 and +0.02 eÅ−2 in
Fig. 2a, represent the negative charge of the O atoms present in the hydroxyl groups of the phospholipids ingredients (phosphate, glycerol, fatty acid). These O atoms
may interact to form hydrogen bonds with the H atoms
in NADESs that produce the peaks between −0.02 and
−0.01 eÅ−2. These peaks belong to the hydroxyl groups
of the sugars, polyols, or acids ingredients of NADESs
and the H atom of ChCl. Likewise, the broad HBD region
of the phospholipids between −0.01 and −0.02 eÅ−2 represent H atoms in the glycol groups of glycerol or in the
functional group of the acids. These may interact with
the elements in the HBA region (which mainly comes
from Cl atoms of ChCl at 0.02 eÅ−2) in Fig. 2b to form
Hayyan et al. SpringerPlus (2016) 5:913
Page 9 of 12
hydrogen bonds;. The importance of these interactions is
perhaps best reflected in the σ-potentials of Fig. 3. The
σ-potential represents the interaction behavior and affinities between molecules in a system. On the σ-potential
plot, a more negative value of µ (σ) indicates higher affinity, and vice versa. Figure 3a shows the σ-potentials of
modelled phospholipids. The phospholipids show clear
and strong affinities for HBD on the left side, and HBA
on the right side, given their high outer negative ranges.
Consequently, they will be more attractive for HBDs and
HBAs of other molecules. Their nonpolar surfaces possess slightly negative values, which promotes average to
a
µ (σ) kcal/molA 2
0
-0.03
-0.02
-0.01
low interactions with other molecules nonpolar surfaces.
NADESs, in contrast, vary slightly in their σ-potentials.
NADES1, NADES2, and NADES3, all possess very strong
affinities for HBDs; relatively weaker affinities for HBAs,
and low affinities for nonpolar surfaces. When compared
to phosphatidylcholine (Fig. 3b), NADES1, NADES2, and
NADES3 affinities for HBD, and nonpolar regions is significantly higher; although relatively similar for HBA. It
entails that these three NADESs can interact strongly
with HBDs, HBAs and nonpolar surfaces of phospholipids than NADES4 and NADES5. These interactions may
correlate with solvent accumulation and aggregation on
σ (eA-2)
0.00
0.01
0.02
0.03
Phosphatidylcholine
Phosphatidylethanolamine
Phosphatidylserine
Sphingomyelin
Glycolipids
-1
-2
-3
-4
Affinity for HBD
Affinity for HBA
Non Polar Region
σ (eA-2)
b
0
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
µ (σ) kcal/molA2
-2
-4
-6
-8
NADES1
NADES2
NADES3
NADES4
NADES5
Phosphatidylcholine
-10
-12
-14
-16
Fig. 3 σ-potentials of phospholipids and NADESs. a Phospholipids σ-potentials, b NADESs σ-profiles with phosphatidylcholine
Hayyan et al. SpringerPlus (2016) 5:913
the cell surface, which ultimately leads to cellular demise
through reduced growth. An example of such critical interactions between groups of opposites polarities
and affinities was shown by Cornmell et al. (2008). The
authors emphasized that the interactions taking place
between aqueous quaternary ammonium salts cations
(such as cholinium cations) and the negatively charged
groups present on cell surfaces may lead to the penetration of the latter in the cytoplasm. The consequences
range from the loss of membrane integrity to a subsequent demise of the cell, through an increased permeability of the cell membrane to exogenous species (Cornmell
et al. 2008).
NADES4 and NADES5 affinities are almost similar to
phospholipids (phosphatidylcholine). These mild affinities of NADES4 and NADES5 -for HBDs and HBAs
of phospholipids-suggest that perhaps their cytotoxic
mechanism is not entirely focused on cellular aggregation, but rather depends on the resulting reactions
engendered by the cellular adsorption. That is, Krebs
cycle stalling and acidosis by NADES5, and cellular
poisoning by NADES4 at a threshold concentration. It
is interesting though to note that the solvents with the
overall lesser affinities are both the most toxic and least
toxic understudied eutectics.
Of course, this model is not an exact replica of what
is found in membranes, especially in terms of ratio and
functional groups occurrence. As such, the higher ratio
of ChCl, or water and sugars in NADES1, NADES2, and
NADES3, and the hypothetical ratio of cell membrane
elements, may explain the resulting fluctuating affinities.
Phospholipids elements especially, consist of a set ratio
of functional groups on the cell surface (carboxyl, phosphate and amino groups). The ratio of these functional
groups dictates the entry and the rate of passage of extracellular materials (such as NADESs’ species) in the intracellular medium, as their proportions differ according
to cell type. These proportions regulate the diffusion of
NADESs, and indirectly affect their effect on the cellular
machinery.
The propensity of NADESs/DESs species to permeate through cellular membranes was suggested to obey
a principle of colloidal biology, which is based on the
Hofmeister phenomenon (Vlachy et al. 2009). An elucidation of the specifics of the principle of affinities
between chaotropic and kosmotropic DESs/NADES species and cell surface groups would provide a strong tool
for the prediction of the toxicity of these mixtures.
Conclusion
NADESs show similar physical characteristics to
DESs. They exhibit high viscosities, poor conductivities and malleable densities at room temperature. These
Page 10 of 12
characteristics are determined by the strong hydrogen
networks holding together their supramolecular structures. Loosening this network brings about ideal conditions for the industrial use of these solvents. Changes in
temperatures alter this network but so does the inclusion
of water as a tertiary component. This study showed that
NADESs are generally less toxic than DESs. Moreover,
it emphasized the significant role of HBDs with regards
to NADESs cytotoxic profiles. The use of biomaterials
appears to be an important asset for lowering their cytotoxicity. Organic acids, as in previous reports, should
be used with caution as they increase the deleterious
attributes of NADESs. The COSMO-RS based computational approach proposed a hypothetical cytotoxic
mechanism of NADESs mostly based on cellular aggregation. Although further assessment is needed to draw a
comprehensive picture of the cytotoxicity mechanism of
these neoteric mixtures; the results obtained in this work
are encouraging with regards to their safety.
Authors’ contributions
MH conceived of the study. MH, YPM, CYL, WFW, and ZS analyzed the data.
MH, YPM, CYL, WFW, ZS, AH, OMA participated in both the interpretation of
results and the preparation of the manuscript. All authors read and approved
the final manuscript.
Author details
1
University of Malaya Centre for Ionic Liquids (UMCiL), University of Malaya,
50603 Kuala Lumpur, Malaysia. 2 Department of Civil Engineering, University
of Malaya, 50603 Kuala Lumpur, Malaysia. 3 Department of Chemical Engi‑
neering, University of Malaya, 50603 Kuala Lumpur, Malaysia. 4 Department
of Medical Microbiology, University of Malaya, 50603 Kuala Lumpur, Malaysia.
5
Department of Pharmacology, University of Malaya, 50603 Kuala Lumpur,
Malaysia. 6 Institute of Halal Research University of Malaya (IHRUM), Academy
of Islamic Studies, University of Malaya, 50603 Kuala Lumpur, Malaysia. 7 UiTM
Medical Specialist Centre, University of Technology MARA, Jalan Hospital,
47000 Sungai Buloh, Selangor, Malaysia.
Acknowledgements
The authors would like to express their gratitude to the University of Malaya
HIR-MOHE (D000003-16001) and to UMRG (RP037B-15AET) for their support
throughout this research.
Competing interests
The authors declare that they have no competing interests.
Received: 27 January 2016 Accepted: 15 June 2016
References
Andersson MP, Jensen JH, Stipp SLS (2013) Predicting pKa for proteins using
COSMO-RS. PeerJ 1:e198
Buggert M, Cadena C, Mokrushina L, Smirnova I, Maginn EJ, Arlt W (2009)
COSMO-RS calculations of partition coefficients: different tools for
conformation search. Chem Eng Technol 32:977–986. doi:10.1002/
ceat.200800654
Bushnell PJ, Boyes WK, Shafer TJ, Bale AS, Benignus VA (2007) Approaches to
extrapolating animal toxicity data on organic solvents to public health.
Neurotoxicology 28:221–226. doi:10.1016/j.neuro.2006.03.013
Choi YH et al (2011) Are natural deep eutectic solvents the missing link
in understanding cellular metabolism and physiology? Plant Physiol
156:1701–1705. doi:10.1104/pp.111.178426
Hayyan et al. SpringerPlus (2016) 5:913
Cornmell RJ, Winder CL, Tiddy GJT, Goodacre R, Stephens G (2008) Accumula‑
tion of ionic liquids in Escherichia coli cells. Green Chem 10:836–841.
doi:10.1039/B807214K
da Costa Lopes AM, João KG, Rubik DF, Bogel-Łukasik E, Duarte LC, Andreaus
J, Bogel-Łukasik R (2013) Pre-treatment of lignocellulosic biomass using
ionic liquids: wheat straw fractionation. Bioresour Technol 142:198–208.
doi:10.1016/j.biortech.2013.05.032
Dai Y, van Spronsen J, Witkamp G-J, Verpoorte R, Choi YH (2013) Natural deep
eutectic solvents as new potential media for green technology. Anal
Chim Acta 766:61–68. doi:10.1016/j.aca.2012.12.019
Dai Y, Witkamp G-J, Verpoorte R, Choi YH (2015) Tailoring properties of natural
deep eutectic solvents with water to facilitate their applications. Food
Chem 187:14–19. doi:10.1016/j.foodchem.2015.03.123
Deliconstantinos G, Villiotou V, Stavrides JC (1995) Modulation of particulate
nitric oxide synthase activity and peroxynitrite synthesis in cholesterol
enriched endothelial cell membranes. Biochem Pharmacol 49:1589–1600.
doi:10.1016/0006-2952(95)00094-G
Domínguez de María P, Maugeri Z (2011) Ionic liquids in biotransformations:
from proof-of-concept to emerging deep-eutectic-solvents. Curr Opin
Chem Biol 15:220–225. doi:10.1016/j.cbpa.2010.11.008
Fernandez-Gomez FJ, Galindo MF, Gómez-Lázaro M, Yuste VJ, Comella JX,
Aguirre N, Jordán J (2005) Malonate induces cell death via mitochondrial
potential collapse and delayed swelling through an ROS-dependent
pathway. Br J Pharmacol 144:528–537. doi:10.1038/sj.bjp.0706069
Ferraz R, Branco LC, Prudêncio C, Noronha JP, Petrovski Ž (2011) Ionic liquids
as active pharmaceutical ingredients. ChemMedChem 6:975–985.
doi:10.1002/cmdc.201100082
Florindo C, Oliveira FS, Rebelo LPN, Fernandes AM, Marrucho IM (2014)
Insights into the synthesis and properties of deep eutectic solvents
based on cholinium chloride and carboxylic acids. ACS Sustain Chem Eng
2:2416–2425. doi:10.1021/sc500439w
Gorke J, Srienc F, Kazlauskas R (2010) Toward advanced ionic liquids. Polar,
enzyme-friendly solvents for biocatalysis. Biotechnol Bioprocess Eng
15:40–53. doi:10.1007/s12257-009-3079-z
Harwood JL, Weselake RJ (2015) Lipid library. American Oil Chemists’
Society. AOCS. http://lipidlibrary.aocs.org/index.cfm. Accessed 25 April
2016
Hayyan M, Mjalli FS, Hashim MA, AlNashef IM (2010) A novel technique for
separating glycerine from palm oil-based biodiesel using ionic liquids.
Fuel Process Technol 91:116–120. doi:10.1016/j.fuproc.2009.09.002
Hayyan A, Ali Hashim M, Mjalli FS, Hayyan M, AlNashef IM (2013a) A novel
phosphonium-based deep eutectic catalyst for biodiesel production
from industrial low grade crude palm oil. Chem Eng Sci 92:81–88.
doi:10.1016/j.ces.2012.12.024
Hayyan M, Hashim MA, Al-Saadi MA, Hayyan A, AlNashef IM, Mirghani MES
(2013b) Assessment of cytotoxicity and toxicity for phosphoniumbased deep eutectic solvents. Chemosphere 93:455–459. doi:10.1016/j.
chemosphere.2013.05.013
Hayyan M, Hashim MA, Hayyan A, Al-Saadi MA, AlNashef IM, Mirghani MES,
Saheed OK (2013c) Are deep eutectic solvents benign or toxic? Chemos‑
phere 90:2193–2195. doi:10.1016/j.chemosphere.2012.11.004
Hayyan A, Hashim MA, Hayyan M, Mjalli FS, AlNashef IM (2014) A new process‑
ing route for cleaner production of biodiesel fuel using a choline chloride
based deep eutectic solvent. J Clean Prod 65:246–251. doi:10.1016/j.
jclepro.2013.08.031
Hayyan M, Looi CY, Hayyan A, Wong WF, Hashim MA (2015) In vitro and in vivo
toxicity profiling of ammonium-based deep eutectic solvents. PLoS One
10:e0117934. doi:10.1371/journal.pone.0117934
Hosoya N, Kawada N (1958) Malonate metabolism in human placenta. J
Biochem 45:363–365
Hosoya N, Kawada N, Matsumura Y (1960) Regulatory effect of malonate
on glucose metabolism in human earlier placenta. J Biochem
48:262–266
Juneidi I, Hayyan M, Hashim MA (2015) Evaluation of toxicity and biodegra‑
dability for cholinium-based deep eutectic solvents. RSC Adv 5:83636–
83647. doi:10.1039/C5RA12425E
Kim Y-S (2002) Malonate metabolism: biochemistry, molecular biology, physi‑
ology, and industrial application. BMB Rep 35:443–451
Klamt A (2005) COSMO-RS: from quantum chemistry to fluid phase thermody‑
namics and drug design. Elsevier, Amsterdam
Page 11 of 12
Klamt A, Eckert F (2000) COSMO-RS: a novel and efficient method for the a
priori prediction of thermophysical data of liquids. Fluid Phase Equilib
172:43–72. doi:10.1016/S0378-3812(00)00357-5
Kuimova MK et al (2009) Imaging intracellular viscosity of a single cell during
photoinduced cell death. Nat Chem 1:69–73. http://www.nature.com/
nchem/journal/v1/n1/suppinfo/nchem.120_S1.html
Laforenza U, Bottino C, Gastaldi G (2016) Mammalian aquaglyceroporin
function in metabolism. Biochim Biophys Acta 1858:1–11. doi:10.1016/j.
bbamem.2015.10.004
Liu KJ-M, Drucker Y, Jarad J (1995) Hepatic glycerol metabolism in tumorous
rats: a 131C nuclear magnetic resonance study. Cancer Res 55:761–766
Liu T, Liu X, Spring DR, Qian X, Cui J, Xu Z (2014) Quantitatively mapping
cellular viscosity with detailed organelle information via a designed PET
fluorescent probe. Sci Rep 4:5418. doi:10.1038/srep05418. http://www.
nature.com/articles/srep05418#supplementary-information
Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J (2000)
Molecular cell biology. W. H. Freeman, New York
Lundholm K, Edström S, Karlberg I, Ekman L, Schersten T (1982) Glucose
turnover, gluconeogenesis from glycerol, and estimation of net glucose
cycling in cancer patients. Cancer 50:1142–1150
Mulyono S, Hizaddin HF, Alnashef IM, Hashim MA, Fakeeha AH, Hadj-Kali MK
(2014) Separation of BTEX aromatics from n-octane using a (tetrabuty‑
lammonium bromide + sulfolane) deep eutectic solvent–experiments
and COSMO-RS prediction. RSC Adv 4:17597–17606. doi:10.1039/
C4RA01081G
Nadiv O, Shinitzky M, Manu H, Hecht D, Roberts CT, LeRoith D, Zick Y (1994)
Elevated protein tyrosine phosphatase activity and increased membrane
viscosity are associated with impaired activation of the insulin receptor
kinase in old rats. Biochem J 298:443–450
Paiva A, Craveiro R, Aroso I, Martins M, Reis RL, Duarte ARC (2014) Natural deep
eutectic solvents—solvents for the 21st century. ACS Sustain Chem Eng
2:1063–1071. doi:10.1021/sc500096j
Pereira MM, Pedro SN, Quental MV, Lima ÁS, Coutinho JAP, Freire MG (2015)
Enhanced extraction of bovine serum albumin with aqueous biphasic
systems of phosphonium- and ammonium-based ionic liquids. J Biotech‑
nol 206:17–25. doi:10.1016/j.jbiotec.2015.03.028
Plagemaen PGW (1971) Choline metabolism and membrane formation in rat
hepatoma cells grown in suspension culture. III. Choline transport and
uptake by simple diffusion and lack of direct exchange with phosphati‑
dylcholine. J Lipid Res 12:715–724
Port AM, Ruth MR, Istfan NW (2012) Fructose consumption and cancer: is
there a connection? Curr Opin Endocrinol Diabetes Obes 19:367–374.
doi:10.1097/MED.0b013e328357f0cb
Qi X-L, Peng X, Huang Y-Y, Li L, Wei Z-F, Zu Y-G, Fu Y-J (2015) Green and efficient
extraction of bioactive flavonoids from Equisetum palustre L. by deep
eutectic solvents-based negative pressure cavitation method com‑
bined with macroporous resin enrichment. Ind Crops Prod 70:142–148.
doi:10.1016/j.indcrop.2015.03.026
Radošević K, Cvjetko Bubalo M, Gaurina Srček V, Grgas D, Landeka Dragičević T,
Radojčić Redovniković I (2015) Evaluation of toxicity and biodegradability
of choline chloride based deep eutectic solvents. Ecotoxicol Environ Saf
112:46–53. doi:10.1016/j.ecoenv.2014.09.034
Ru C, Konig B (2012) Low melting mixtures in organic synthesis—an
alternative to ionic liquids? Green Chem 14:2969–2982. doi:10.1039/
C2GC36005E
Ru J et al (2015) Morphology-controlled preparation of lead powders by
electrodeposition from different PbO-containing choline chlorideurea deep eutectic solvent. Appl Surf Sci 335:153–159. doi:10.1016/j.
apsusc.2015.02.045
Santos CR, Schulze A (2012) Lipid metabolism in cancer. FEBS J 279:2610–2623
Smith EL, Abbott AP, Ryder KS (2014) Deep eutectic solvents (DESs) and their
applications. Chem Rev 114:11060–11082. doi:10.1021/cr300162p
Sowmiah S, Srinivasadesikan V, Tseng M-C, Chu Y-H (2009) On the chemical
stabilities of ionic liquids. Molecules 14:3780
Tang B, Row K (2013) Recent developments in deep eutectic solvents
in chemical sciences. Monatsh Chem 144:1427–1454. doi:10.1007/
s00706-013-1050-3
Ullah Z, Bustam MA, Man Z (2015) Biodiesel production from waste cook‑
ing oil by acidic ionic liquid as a catalyst. Renew Energy 77:521–526.
doi:10.1016/j.renene.2014.12.040
Hayyan et al. SpringerPlus (2016) 5:913
Vlachy N, Jagoda-Cwiklik B, Vácha R, Touraud D, Jungwirth P, Kunz W (2009)
Hofmeister series and specific interactions of charged headgroups
with aqueous ions. Adv Colloid Interface Sci 146:42–47. doi:10.1016/j.
cis.2008.09.010
Wagle DV, Zhao H, Baker GA (2014) Deep eutectic solvents: sustainable media
for nanoscale and functional materials. Acc Chem Res 47:2299–2308.
doi:10.1021/ar5000488
Wen Q, Chen J-X, Tang Y-L, Wang J, Yang Z (2015) Assessing the toxicity and
biodegradability of deep eutectic solvents. Chemosphere 132:63–69.
doi:10.1016/j.chemosphere.2015.02.061
Page 12 of 12
Wu B-P, Wen Q, Xu H, Yang Z (2014) Insights into the impact of deep eutectic
solvents on horseradish peroxidase: activity, stability and structure. J Mol
Catal B Enzym 101:101–107. doi:10.1016/j.molcatb.2014.01.001
Zhang Q, De Oliveira Vigier K, Royer S, Jerome F (2012) Deep eutectic solvents:
syntheses, properties and applications. Chem Soc Rev 41:7108–7146.
doi:10.1039/C2CS35178A
Zhao B-Y, Xu P, Yang F-X, Wu H, Zong M-H, Lou W-Y (2015) Biocompatible
deep eutectic solvents based on choline chloride: characterization and
application to the extraction of rutin from Sophora japonica. ACS Sustain
Chem Eng 3:2746–2755. doi:10.1021/acssuschemeng.5b00619